1. Field of the Invention
The present invention relates to a method and an apparatus for ultrasonic wave measurement, and more particularly, to a method and an apparatus for ultrasonic wave measurement suitable for measuring a velocity of sound propagated through a medium subject to measurement such as a gas, a liquid, a solid or the like. Further, the present invention relates to the method and apparatus for ultrasonic wave measurement suitable for determining a component, a concentration and a modulus of elasticity or the like in the case where the medium subject to measurement is a gas or a liquid, and for determining a modulus of elasticity, strength, fatigue, stress, history, life or the like in the case where the medium subject to measurement is a solid.
2. Description of the Related Art
According to a method of measuring the velocity of sound by an ultrasonic wave, an ultrasonic wave transmitter section transmits an ultrasonic wave into a medium subject to measurement, and then, an ultrasonic wave receiver section receives a reflection signal generated on the basis of the transmitted wave, and thus, an absolute velocity (V) of sound propagated through the medium subject to measurement is determined on the basis of a propagation time (t.sub.t) and a propagation distance (L) determined by the transmitted wave.
Conventional methods of measuring the velocity of sound include a sing around method, an overlap method and the like.
The sing around method has been used resulting from demands for its stability of measurement, automatic measurement or the like. In this sing-around method, in order to measure a propagation time (Ts) (see FIG. 4), the following equation (1) is required. EQU T.sub.s =2t.sub.e +2t.sub.a +2t.sub.t (1)
Where,
t.sub.e : electrical delay time PA1 t.sub.a : transmission plate propagation time PA1 measuring a time t.sub.1 until an ultrasonic wave transmitted from the first ultrasonic sensor is received in the second ultrasonic sensor, a time t.sub.2 until the ultrasonic wave transmitted from the first ultrasonic sensor is reflected by the second reflector, and is received by the first ultrasonic sensor, a time t.sub.3 until the ultrasonic wave transmitted from the second ultrasonic sensor is received in the first ultrasonic sensor, and a time t.sub.4 until the ultrasonic wave transmitted from the second ultrasonic sensor is reflected by the first reflector, and is received in the second ultrasonic sensor, within a medium subject to measurement; and PA1 determining a propagation time t.sub.t of an ultrasonic wave in the distance L within the medium subject to measurement from the following equation t.sub.t =(t.sub.4 -t.sub.3 +t.sub.2 -t.sub.1)/2 and an absolute sound velocity V of the medium subject to measurement from the following equation V=L/t.sub.t.
Therefore, this method includes times T.sub.e and T.sub.a other than the truly necessary propagation time (t.sub.t). The transmission plate propagation time (t.sub.a) is sensitive to the influence of external factors such as temperature, pressure or the like. On the other hand, the electrical delay time (t.sub.e) has a different inference for each electric circuit system and each component; for this reason, it is difficult to obtain high accuracy and inter-changeability.
According to the overlap method, by measuring multiple wave propagation times t.sub.01 and t.sub.02 (see FIG. 5) of a multiple reflection signal successively generated on the basis of a transmission wave transmitted from the ultrasonic wave transmitter section in a medium subject to measurement held between a transmission plate and a reception plate facing each other, a propagation time (t.sub.t) is obtained by the following equation (2). EQU t.sub.t =(t.sub.02 -t.sub.01).div.2 (2)
However, in fact, it is difficult to arrange and manufacture a reflector and the transmission plate in a fully parallel state, and its parallelism varies depending upon pressure or an external force. As a result, measurement error is caused by an imperfection in the parallelism and an angle of these plates.
Moreover, in the case where a reflectivity of the reflector is small, according to the method using the multiple wave, a signal energy of the received signal is small. For this reason, measurement is unstable. As a result, there may be a case where no measurement is made.